Sword of Spirit
Legend
So it seems like adding a movement cost of 1 for each allied square passed through (and the inability to end in an occupied space as usual) should decrease the conga line effect while still permitting split movement.
This is the winner, I think. This is a simple extension of the "difficult terrain" rule, and would reduce the issue with the minimum complexity added to the rules.
"Moving through another creature's space counts as moving through difficult terrain."
One could almost make an argument that the currently vague rules for difficult terrain can already support that interpretation.